1 Field of the Invention
The present invention relates to water treatment and particularly to reduction of the water treatment costs and water requirements of thermal power stations. More specifically, the present invention is directed to systems for the treatment of makeup water for the cooling and steam circulation systems of a thermal power station and also to apparatus for reducing undesirable discharges from such a power station. Accordingly, the general objects of the present invention are to provide novel and improved methods and apparatus of such character.
2 Description of the Prior Art
While not limited thereto in its utility, the present invention is particularly well suited for use in and as a process for the preparation of makeup water for the cooling water and steam circulation systems of an electrical power generation station while employs a fossil fuel fired steam generator. Such a power station will conventionally include condensing type turbines, wet cooling towers, a flue gas desulfurizer and an ash removal system. It is necessary, in the operation of such a power station, to provide makeup water for the cooling system and steam circulation systems and to treat the various waste water streams which result from plant operation.
The necessary cooling system makeup water for thermal power stations with wet recooling, which is almost exclusively derived from surface water available in the plant vicinity, must be mechanically and chemically treated so that it meets the plant operational requirements and also meets existing regulations with respect to the contaminant level of waste water discharges.
The state of the art of power station water treatment is believed to be exemplified by the informational brochure issued by the Raginisch Westfalischen Elektrizitatswerke Aktiengeselesschieft and describing the Neurath power station. This power station employs coal as its fuel. The requisite raw water for plant operation is stored in reservoirs and, prior to use, is passed through a decarbonizing station. The cooling water system of the power station is supplied with makeup water directly from the decarbonizing station. A part of the decarbonized water flow is pumped to a full demineralization station and, after passing through gravel, cation, anion, and mixed bed filters, serves as makeup water for the steam systems of the power station. The power station also includes apparatus which collects and desalinates the unusable condensate from various plant apparatii. The desalinated condensate and the makeup water from the demineralization station provides feed water for the power station condensate storage tanks.
Systems of the type briefly described above have the disadvantage that large quantities of waste water containing different amounts of soluble salts and undissolved solid substances are produced at different points within the power station. These different waste water streams have, for the most part, been purified in suitable separate water treatment units, thereafter discharged to drainage systems and usually find their way into the surface water. Thus, in the case of a thermal power station with circulation cooling; i.e., wet cooling towers; and flue gas desulfurization, the following differently contaminated waste water streams must be treated and discharged:
(1) Water resulting from the cooling system blowdown with a high salt content and sometimes with free chlorine and suspended substances. PA1 (2) Regenerate from the steam generator water treatment; i.e., regenerate resulting from a full demineralization and condensate treatment; with high salt content and an excess of acids or alkalines. PA1 (3) Waste water from the flue gas desulfurizer with high salt content and a high concentration of solids. PA1 (4) Waste water or sludge from the makeup water treatment system for the cooling tower circulation system. PA1 (5) Filter washing water from the steam generator feed water treatment system and/or pre-cleaning units for makeup water treatment as well as eventual suspended substances from the cooling circulation system filtering plant for the individual water streams. PA1 (6) Waste water from the ash cooling system with relatively high salt content and suspended substances. PA1 (a) When an excess of acid or alkali is present, neutralization will be performed in suitably large storage and neutralization basins and the neutralized water is discharged without the filterable substances resulting from the neutralization being removed therefrom. PA1 (b) When the waste water contains precipitatable substances, sedimentation will be allowed to occur in suitable sedimentation basins, the clear water will be discharged and the precipitated sludge will either be directly dumped or mechanically dewatered and subsequently dumped. PA1 (c) If the waste water has a high salt content, but does not contain either an excess of acids, alkalines or sediment, it is directly discharged. PA1 (d) No effort is presently made to reduce the amount of ammonium or hydrazine in waste water containing these substances prior to dumping. PA1 V=Evaporation losses (t/h) PA1 S.sub.z =Concentration of the makeup water (g/t) PA1 S.sub.k =Limiting concentration in the circulation system (g/t) PA1 (a) Permeate from the reverse osmosis unit which treats the concentrate from the cooling tower circulation system blowdown water treatment stage. PA1 (b) Permeate from a reverse osmosis unit, if any, connected in series with the reverse osmosis unit mentioned in paragraph (a) above. PA1 (c) Condensate from the evaporator used for concentrating high salt content waste water streams comprising: PA1 (1) Concentrate from the reverse osmosis unit which treats the concentrate from the cooling tower circulation system blowdown water treatment stage. PA1 (2) Waste water from a heavy metal removal stage which receives, after detoxification and ammonium concentration, the regenerate from the steam generator feed water treatment unit. PA1 (3) Regenerate from the softening stage of the flue gas desulfurization waste water treatment system, when required, together with the concentrate from a downstream reverse osmosis unit in this desulfurization waste water treatment loop. PA1 (a) Waste water and all process water containing salts are, for all practical purposes, completely used by means of suitable internal circulation systems. PA1 (b) Primarily through the use of physical concentration technology, the readily available and comparatively inexpensive energy sources at a thermal power station, such as electrical power and waste heat, are employed to achieve economical circulation of the individual water streams. PA1 (c) Environmental pollution caused by waste water from a thermal power station is drastically reduced since almost all water is recirculated and only depositable solid substances must be discarded. PA1 (d) The physical size of the steam generator feed water treatment plant may be significantly reduced through joining low salt content streams together for delivery to the feed water treatment apparatus. Thus, in accordance with the present invention demineralization of high salt content raw water through the use of costly multi-stage full demineralization technology is avoided and a condensate treatment unit, which in any event would be available, is used for plant condensate treatment.
(a) In the case of decarbonization with ion exchange, this waste water has a high salt content and an excess of acids. PA2 (b) In the case of decarbonization with lime alkali reacting sludge, this waste water has solid contents between 20 and 70% according to the de-watering process used.
In addition to the above listed waste water streams, quantities of waste water which contain a high level of solids as well as chemical contaminants are periodically produced. By way of example, when inspections occur piping and steam generator flushing waste water will be produced. Similarly, chemically contaminated waste water, either acid or alkaline, will result from steam generator pressure tests, cleaning procedures, etc. In comparison to the waste water quantities from the above-enumerated sources, however, the quantity of the intermittently occurring waste water is sufficiently small that it can be ignored.
When the alkaline method of operation is employed, the washing and regeneration water from the condensate treatment apparatus of the power station contains ammonium and hydrazine in differing quantities.
The state of the prior art with respect to treatment of the above listed individual waste water streams is as follows:
The largest single contributor to the total volume of waste water discharged from a power plant will comprise the blowdown water from the cooling tower circulation system. This blowdown water will typically have a high total salt content including a high concentration of chloride and sulfate.
A process for reducing the cooling water requirements of a thermal power station through the use of reverse osmosis has been described in the literature. In this process concentrated circulation water; i.e., blowdown water; is passed through a reverse osmosis unit, demineralized and the permeate is then returned to the cooling tower circulation system. As a result of the recycling of the permeate, which has a reduced salt content, the salt content of the water mixture comprising the permeate and treated water from a makeup water treatment system will correspondingly be reduced. Furthermore, some of the permeate from the reverse osmosis unit may be fed to the steam generator feed water treatment plant as pre-treated (demineralized) water. The previously known process employing reverse osmosis has been limited to application to the cooling circulation system, and to some extent to the steam generator feed water treatment system, and has had a limited effect insofar as reduction of the water costs incident to operation of a thermal power station.